For connection between fluid pipes, a double ferrule-type fitting is a fitting that provides a high sealing property with a small tightening torque (see PTL 1 and PTL 2).
FIGS. 1A to 1C are views illustrating a typical double ferrule-type fitting. Referring to FIG. 1A, a double ferrule-type fitting 90 includes a fitting body 91, a nut 92, a front ferrule 93, and a back ferrule 94.
The fitting body 91 and the nut 92 are coupled together by screw coupling. As the screw coupling is tightened, the nut 92 pushes the back ferrule 94 (point a in the figure) as shown in FIG. 1A. The back ferrule 94, pushed by the nut 92, pushes the front ferrule 93 (point b in the figure) while sliding under the front ferrule 93 as shown in FIG. 1B. As the back ferrule 94 slides under the front ferrule 93, the front ferrule 93 has its tip bite into a pipe 95 along a tapered surface of the fitting body 91 while rotating to rise so as to raise the rear end (point c in the figure).
Then, as the front ferrule 93 rises, with point b serving as the point of effort and point d as the fulcrum, as shown in FIG. 1C, a strong biting force is generated at point c as the point of load, thus realizing a good seal mechanism.
With such a double ferrule-type fitting 90, it is possible to substantially prevent a fluid leakage because of its high sealing property.
On the other hand, various single ferrule-type fittings have been proposed in order to reduce the cost by reducing the number of parts and to facilitate the installation (see PTL 3 to PTL 5). A single ferrule-type fitting is a fitting having a single ferrule, and includes a fitting body, a nut, and a ferrule. The ferrule, being pushed by the nut as the screw coupling is tightened, has its tip bite into the pipe along a tapered surface of the fitting body, thus realizing a seal mechanism. Using a single ferrule placed between the fitting body and the nut reduces the number of parts and also facilitates the installation.
These single ferrule-type fittings obtain a force for making the tip of the ferrule bite into the pipe in the radially inward direction from the force of the nut pushing the ferrule in the axial direction. The shape of the single ferrule is devised so as to convert the direction of force from the axial direction to the radially inward direction.
However, as opposed to a double ferrule-type fitting, where as the back ferrule 94 slides under the front ferrule 93, the front ferrule 93 rises, with point b serving as the point of effort and point d as the fulcrum, a single ferrule-type fitting changes the direction of force by the shape of the single ferrule, and it is therefore not possible to efficiently generate a force in the radially inward direction generally at the tip of the pipe.
PTL 6 provides various discussions on the characteristics of single ferrule-type fittings.